Dielectric testing device



May 27, 1958 K. H. WEBER 2,836,792 DIELECTRIC TESTING DEVIC Filed Jan. 30, 1956 Fl 4b' frenr United States 2,835,792 nrntncrnic restino navicn Kurt H. Weber, Pittsfield, Mass., assignor to General Electric Company, a corporation of New "loris T he present invention relates to dielectric testing de vices, and more particularly to an apparatus for determining the breakdown strength of dielectric liquids.

Known insulating liquid testing devices of the above type have not proved satisfactory, in that they did not accurately reflect the voltage breakdown characteristics of the liquid being tested. Further, in a known standard type of dielectric gauge, the field produced by the electrode gap structure does not correspond closely enough to the more nearly uniform field conditions characteristic of liquid filled electrical apparatus presently being used, particularly in the case of transformers.

As a result, it has been found that the correlation between the insulating oil tests as carried out by known standard and otherl methods and the actual failure rate of the electrical devices in which the oil was used was not sufficiently close to provide a reliable indication of the suitability of the dielectric liquid tested. The prior testing arrangements apparently did not take into account the variations in dielectric strength of any particular oil sample and these devices tended to minimize the real differences in oil quality and oil strength.

It is an object of the present invention to provide an insulating liquid test device which overcomes the disadvantages of prior art gauges.

It is another object of the invention to provide a dielectric test device of the above type for readily detecting lluctuations in the quality of the oil or other liquid dielectrics to be used in electrical apparatus such as transformers.

It is another object of the invention to provide an insulating liquid testing device which indicates a practical end point for operations designed to improve oil quality, such as blending, drying, deaerating and filtering It is still another object of the invention to provide a testing device of the above type which simulates the more critical electric field configurations within the apparatus in which the oil is incorporated to such a degree that failures in the apparatus due to inadequately high quality of the liquid dielectric may be predicted and avoided.

lt is a-further object of the invention to provide a dielectric testing device by means of which a representative measurement may be obtained'of both the mean Value of the liquid dielectric strength and the variation of values Vfrom the mean which can be expected in actual use of the dielectric.

With the above objects in view, the present invention comprises a device for testing the dielectric strength of insulating liquids and the like, which comprises the cornbination of a vessel in which the insulating liquid is adapted to be contained, a pair of electrodes in the vessel having effectively spherical end portions facing each other and arranged with its spherical end portions spaced a predetermined distance from each other to provide a gap therebetween, and means for circulating the insulating liquid in the vessel through the gap. T he atent O ice 2 circulating means in a preferred embodiment of the invention comprises an impeller disk arranged above the gap and driven by a vertical shaft rotated by a motor suitably mounted on the cover of the test vessel.

The invention will be better understood from the following description taken in conjunction with the accompanying drawing, in which:

Fig. l is an elevational view partly in section showing a preferred embodiment of the present invention;

Fig. 2 is a detailed fragmentary view of the cover structure of the device shown in Fig. l;

Fig. 3 is a diagram of an electrical circuit which may be used in the operation of the present device.

Fig. 4a graphically illustrates the correlation between test results from the present dielectric gauge and performance of electrical units using the dielectric tested; and

Fig. 4b is a graphical showing similar to that of Fig. 4a and relating to results obtained by a prior art gauge.

Referring now to the drawings, and particularly to Fig. l, there is shown a container or test cell 1 made of a suitable material, such as methacrylate resin or the like, which is adapted to contain the insulating liquid or oil 2 under test. The container is preferably transparent as shown in order that the condition of the oil and the operation of the test may be viewed as the test proceeds. However, if desired, the container may be made of a suitable opaque insulating material. Preferably, the container is at least of a one quart capacity which is in contrast to the volume of known standard test devices which is usually about 1/6 of a quart. Electrodes 3 and 4 are threadably mounted in opposite walls of container 1 by means of sleeves 5 and 6, respectively. Knurled nuts 7 and 8 are provided for suitable adjustment of the spacing of electrodes 3, 4. The electrodes are provided with hemispherical end caps 9, 10 with rounded edges and are arranged facing each other and spaced apart a predetermined distance to provide the desired gap width. A cover 11 closes the top of the container 1, the cover having supporting members 12 and 12' on which is mounted a liquid circulating device 13 comprising a motor 14- driven by any suitable electrical or other means, a shaft 15 driven by the motor, an impeller 16, and an impeller shaft 16 coupled to shaft 15 and passing through an oversize aperture in cover 11. As shown, the impeller is in the form of a radially slit disk with one side of the slit bent away from the plane of the disk.

As more clearly shown in Fig. 2, cover 11 is provided with a wedge-shaped portion 17 on its underside which operates to outwardly displace excess liquid and air from the top of the liquid filled container 1 when it is set in place at the opening of the container. The speed of motor 14 and consequently the speed of impeller-l is controlled as desired by any suitable electrical or other means (not shown).

Electrodes 3 and ar: connected to a suitable source of voltage by means of which the voltage can be raised until breakdown occurs through the gap between the end portions 9, 10. Fig. 3 is an illustration of a circuit which may be satisfactorily used to provide the voltage desired. As shown in Fig. 3,' there is provided a variable ratio auto-transformer 13 connected to a suitable source of line voltage 19, e. g. of il() volts A. C., and a testing transformer 20 comprising primary and secondary windings, and a tertiary winding connected to a grounded voltrneter 2l to indicate the voltage at which the gap breaks down, the electrodes being connected to the secondary winding of the testing transformer.

As shown in the drawing, it is preferred that the impeller shaft extend vertically through -the cover 11 and into the liquid-filled container 1. For best Aresults the impeller disk 16 is arranged about half way between the top of the electrodes and the bottom of the cover, care being .taken that the disk i6 is not so close to theV gap Vas toldistort the electrical field therein. In the arrangement, show-n, `the actionof the rotating impeller .i6 .provides :sev-

eral advantages. The impelling action `of disk l5, rotating as shown in Fig. 2, circulates-.the .oil in the `.vessel as shownbygthe arrows `rin .Fig l, it meves'.the liquid -upwardly through the gap betweenrthe electrodes and at the same time it draws any gaseous .breakdown products upwardly .toward `the -top of the .container from which it escapes through the .oversize aperture provided for rentry ofthe impeller shaft. Asa result, .the oil sample is Vfully explored and the succeeding breakdown tests of thesame sampleare not aected by the presence of .the breakdown products. Also, due .to the arrangement shown it is not necessaryto provide an oil seal .around the .impellershaft to prevent .escape .of .the liquid under test. Further, the illustrated construction of a combined cover and .impeller assembly unit facilitates mounting of the impeller motor and enables ready assembly and disassembly ofthe entire structure.

While the electrode caps are shown as of hemispherical form, ,it will be .understood that other spherical forms, including. substantially complete spheres or segments thereof could be used in accordance with the invention. In .addition .to the .advantages relating to closer correspondence to actual eld conditions in the large-scale electrical devices which -the testV cell is designed to simulate, the spherical electrode form also facilitates 'proper adiustment of the gap spacing with any'orientation of the electrode shanks. -For example, where substantially completelyspherical caps are employed, the electrode Shanks may -be arranged at right angles to .each other, and the gap .conditions would still be substantially the Vsame as inthe illustrated arrangement, an-advantage which obvious'ly vCould not be obtained in the case of the prior parallel-plate type of electrodes.

.In the operation of the dev ice described, the .entire sample .of the liquid 2 to .be evaluated, e. g., mineral oil, is vput into the container 1 at one time, the vessel being lled to the top. The cover i1 with .attached impeller apparatus 13 is placed over the vessel opening in .such manner lthat the excess insulating liquid .is removed .by the .wedge i7, as `already indicated. The `motor V14 is then started to cause circulation of the .oil ,in the .container at auniforrn speed to thereby assure that representative portions of ythe dielectric .liquid flow through the gap. Voltage. is .applied to .the electrodes by an initial -setting of the auto-transformen'and by adjustment of the .latter device, which is preferably motor driven, the .voltage across the gap is raised vat a constant vrate untilV .the gap breaks down, .a condition which is noted from .the voltmeter when the needle suddenly drops from its peak value .orV by .observation o f the test cellor other manner.

In determining the .breakdown characteristics ofthe oil sample a series of .such tests is made and the various breakdown values are noted. From these values, the mean breakdown value and other significant statistical parameters, such as the coeiicient of variation, are obtained, and from these data the suitability of the dielectric material is ascertained. From the variationdata, for example, particular defects in the dielectric properties of the oil can Vbe determined moreY sensitively than by the mean value. Y Y

It has been found, as shown hereinafter, that the voltage readings obtained'from the described arrangement areparticularly accurate in showing theV dielectric strength of the liquid being tested, especially insofar as it relates to the electrical failure characteristics infwhich the oil is to be used.

In a `typical construction of the present device, the volume of the sample container is about 1 quart, the i1npeller disk is about ,t2/15 of an inch in diameter, `and the radius of -the hemispherical electrodes is about 1 inch.

of the apparatus each unit.

y in. .the

. that the arrangement of 4 Y .Y These dimensions, however, are not critical. The gap spacing, for example, may be 8G mils although this spacing may be varied depending upon the voltage source available. It has been found, however, that the gap spacing should not exceed 8O mils normally because with this spacing or less it has been found that the device is more sensitive to solid contaminants in the insulating liquid.

' Tests made of various types of electrode shapes showed conclusively that the' vvariation of test devices in sensitivity to quality of the liquid dielectric varied VVgreatly with the eiectrode shape. It has been found .asa-result of' these tests that the present hemispherically capped electrodes when used within a suiicientlyflargevolumerof oilV circulated by an impeller as described represented more accurately the electrical strength characteristics of apparatus utilizing liquid dielectrics such as transformers.

The configuration of the electrodes as shown, particularly when taken in conjunction with theirnpeller action as described, provides several advantages .over the prior;

art test devices. The'confguration of the gap between.

these yspherical electrodes .is such that the oil is allowed e to pass freely therethrough, in contrast to thecondition which exists in the case of the prior devices where .sharp,

edged parallel plate electrodes are used. inthe latter. type of device, dueto the fieldgenerated, .thebreakdown products in the oil Y are confined within Ithe space'between lthe electrodes. inV addition, the eld generated ,by .the

parallel-.plate electrodes does. not.' approximate thel .unie

form field conditions now-encountered .in present day. transformer apparatus. The provision of the impeller arrangement .as shown' a-llows exploration of ftheentire. .sample .in the container, it takes into account effects .produced vb y .solid .contaminants in the sample, ,andj provides for cleaning out all breakdown products .from between the electrodes. Y lt has been found,.accor.ding'ly,

impeller and the particular electrode coniiguration as shown is such as Yto .provide opti-, mum .results in the indication of breakdown characteristics of the insulating liquid underV test.

Figs. 4a and 4b .show the results obtained in a .series of comparative tests demonstrating the improved reliability of the present dielectricgau'ge as Vcomparedto the standard gauge commonly usedin the industry.. Figs. 4a and 4b show. the correlation between high and low brea'kdown voltage of oil samples, and the corresponding .elect tricalfailure of large-,scale electrical devices inwhich the oil was a majorY insulation component. lnthese. figures the breakdown voltage in kilovolts as measured Lby the respective gauges is plotted against the number of electrical units tested as represented by the number; of oil samples lhaving Y'the Vsamebreakdovvn values.- Fig. 4a,`

shows the 'results obtained bythe present dielectric gauge,

injtest and the .units with simple` hatching are those which,

while. not. failing in test, had certain electrical character-V isticsnotup to desired levels. In these tests, the same oil samples rfrom the same A19.

electrical devices were given dielectric tests in'both the present gauge and the prior standard gauge to obtain the results shown, it being understood that the failure Y.data in .both .Figures 4a andl 4b. for .the 19 electrical devices was Vobtained lfrom the same electrical strength test .for

Comparing Fig.' 4a with Fig, 4b, it will be seenthat lin every ycase where the present .gauge showed a .highKV Y level of breakdown strength in the oil tested, theelectrical devices whichincorporated the oil also showed aqcorre spondingly high electrical strength. The appearance-pf failledunits .in thecase of oil testedV bythe standard gauge where the .gauge Vtest indicated a .dielectric'strength atfthe higher portion of the breakdown voltage scale shows 1inadequatel correspondence between gaugeftes't .resultsfand practical performance. This unreliability is-shownfalso assays.,

by the occurrence of unit failure at breakdown voltages equal to or even considerably higher than those of units which did not fail in test.

Of particular significance in this connection is the much greater resolution, i. e., spread, in terms of breakdown voltage atlorded by the present gauge, as shown by the Fig. la graph compared to that of Fig. 4b. Due to the greater resolution and the distinct demarcation between satisiactory and unsatisfactory samples now made possible, much more certain and ready determination can be made of the performance characteristics of largescale electrical units on the basis of rest ts obtained by the dielectric testing device.

it will be evident, therefore, that the present device more accurately simulates the more critical field conditions within the transformer or other electrical apparatus than the prior known dielectric gauges and accomplishes tlts to such a degree that failures due to insuiciently high quality of the liquid dielectric may be readily foreseen and necessary measures taken to improve it.

While the invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the scope of the invention. Therefore, the appended claims are intended to cover all such equivalent variations as come within the true spirit and scope of the invention.

What l claim as new and desire to secure by Letters Patent of the United States is:

1. Apparatus :for testing the dielectric strength of insulating liquids comprising, in combination, a Vessel for containing a predetermined amount of the insulating liquid, a pair of eectively spherical electrodes in said vessel spaced a predetermined distance from each other to provide a gap therebetween, and means for circulating the entire predetermined amount of the insulating liquid in said vessel through said gap.

2. Apparatus for testing the dielectric strength of insulating liquids comprising, in combination, a vessel for containing a predetermined amount of the insulating liquid, a pair of electrodes in said Vessel having spherical end portions spaced a predetermined distance from each other to provide a gap therebetween, and means cornprising a liquid impelling member for circulating the entire predetermined amount of the insulating liquid in said vessel through said gap.

3. Apparatus Ifor testing the dielectric strength of insulating liquids and the like comprising, in combination, a vessel for containing a predetermined amount of the insulating liquid, a pair of electrodes in said vessel and having spherical end portions facing each other, said spherical end portions being spaced along an axis a predetermined distance frorn each other to provide a gap therebetween, and means for circulating the entire predetermined amount of the insulating liquid in said vessel through said gap, said circulating means comprising a liquid impelling member in said vessel spaced from said gap and rotatable about an axis transverse to said firstmentioned axis.

4. Apparatus for testing the dielectric strength of in sulating liquids comprising, in combination, a Vessel for containing a predetermined amount of the insulating liquid, a pair of electrodes in said vessel having spherical end portions spaced along an axis a predetermined distance from each other to provide a gap therebetween, and means tor circulating the entire predetermined amount of the liquid in said vessel through said gap comprising a liquid impelling member in said Vessel spaced from the gap and being rotatable about an axis normal to said iirstmentioned axis, the axis of said rotatable member passing approximately midway between said spaced electrode end portions.

d 5. Apparatus for testing the dielectric strength of insulating liquids comprising, in combination, a Vessel for containing a predetermined amount of the insulating liquid, a pair of electrodes in said vessel having spherical end portions spaced a predetermined distance from each other to provide a gap therebetween, means for circulating the entire predetermined Aamount of the insulating liquid in said vessel thro-ugh said gap, and means for applying an electrical potential between said electrodes.

6. Apparatus for testing the dielectric strength of insulating liquids comprising, in combination, a vessel or containing a predetermined amount oi the insulating liquid, a pair of electrodes in said vessel having spherical end portions spaced a predetermined distance from each other to provide a gap therebetween, means for circulating the entire predetermined amount of the insulating liquid in said vessel through said gap, means tor applying varying electrical potential between said electrodes, and means for indicating the breakdown voltage of the insulating liquid in said vessel.

7". paratus for testing the dielectric strength of insulating liquids compr' ing, in combination, a vessel in which the insulating tid is adapted to be contained, said vessel being open at its top; a pair oi electrodes in said vessel having spherical end portions spaced a predetermined distance irom each other to provide a gap therebetween; and a combined cover and liquid impeller assembly unit mounted at the top of said vessel for enclosing the same and for circulating the insulating liquid therein through said gap.

8. Apparatus for testing the dielectric strength or insulating liquids Comui ing, in combination, a vessel in which the insulating liquid is adapted to be contained, said vessel being open at its top; a pair of electrodes in said vessel having spherical end portions spaced a predetermined distance irom each other to provide a gap therebetween; and a combined cover and liquid impeller assembly unit mounted at the top of said vessel for enclosing the same and for circulating the insulating liquid therein through said gap, said assembly unit comprising a cover member, an elongated member extending through said cover member and having an impelling member at its lower end extending transverse thereto and arranged spaced above said gap, and driving means mounted on said cover member and connected to said elongated inember for driving said impelling member.

9. An apparatus as defined in claim 8, wherein said cover member is formed with an aperture through which said elongated member passes with substantial clearance to allow escape of gaseous breakdown products from said vessel.

l0. An apparatus as defined in claim 9, wherein the underside of said cover member is inclined to provide for outward displacement of excess insulating liquid and air from the top of said vessel during mounting of said assembly unit on said vessel.

11. Apparatus as dened in claim 1, in which the electrodes are normally in horizontal alignment and the direction of circulation of liquid through said gap is ver* tically upward.

Reierences Cited in the file of this patent UNITED STATES PATENTS 

